The present invention relates to a method for etching a tapered bore into a silicon substrate from a first face thereof, and the invention also relates to a semiconductor wafer comprising a substrate layer having a tapered bore therein.
Micro-machined components formed in a silicon device layer of a semiconductor wafer, in general, are formed in a relatively thin silicon device layer, which is supported on a handle layer. The device layer in which the micro-machined components are to be formed is laminated to the handle layer, which. In general, is also of silicon. In general, an oxide layer is formed between the handle layer and the device layer. The handle layer provides support to the relatively thin device layer within which the micro-machined components are formed. The oxide layer forms an electrical insulation barrier between the device layer and the handle layer. In general, it is necessary to be able to access such micro-machined components through the handle layer, and this requires the formation of access bores extending through the handle layer to the respective micro-machined components. In general, it is desirable that the access bores to such micro-machined components should be accurately aligned with the corresponding one of the micro-machined components, and additionally, it is desirable that the access bores should be accurately dimensioned, and in particular, the transverse cross-sectional area of such bores should be of accurate dimensions. For example, where it is desired to terminate an optical fibre extending through an access bore adjacent the corresponding micro-machined component, it is important that as well as being accurately aligned with the micro-machined component, the access bore should be accurately dimensioned in order to positively and accurately secure and locate the optical fibre relative to the micro-machined component. It is also desirable that such access bores be dimensioned to form a relatively tight fit around to the corresponding optical fibre in order that when the optical fibre is tightly located in the access bore, t e terminal end of the optical fibre is accurately aligned with the micro-machined component. In general, axial alignment of such access bores relative to the corresponding micro-machined component can be achieved without too much difficulty. However, the etching of such access bores of relatively accurate dimensions, particularly relatively accurate cross-sectional dimensions, presents considerable difficulties, and thus subsequent alignment problems when locating the optical fibre in the access bore relative to the micro-machined component.
Additionally, due to the fact that the optical fibre should be a relatively tight fit, and preferably, an interference fit in the access bore. It is desirable that a tapered lead in should be provided to the bore for facilitating initial insertion of the optical fibre into the access bore. This also is difficult to achieve with any degree of accuracy.
In known methods for forming such access bores, an anisotropic wet etch is used where the etchant may, for example, comprise a mixture of potassium hydroxide, isopropylalcohol and water. In general. It is difficult to control the cross-sectional shape of an access bore in such wet etch processes. In particular, it is difficult to wet etch such access bores of regular circular cross-section. This is due to the fact that wet etches tend to etch along the crystalline plane of silicon, and typically, attempts to etch bores of circular cross-section tend to result in bores of square or rectangular cross-section. This is so irrespective of the etch opening formed in a mask through which the etchant is being directed at the silicon.
There is therefore a need for a method for etching a bore, and in particular, a tapered bore into a silicon substrate which overcomes these problems.
The present invention is directed towards providing such a method, and the invention is also directed towards providing a semiconductor wafer comprising a substrate layer having a bore etched therein by the method according to the invention.
According to the invention there is provided a method for etching a bore into a silicon substrate from a first face thereof, the method comprising the steps of:
forming a masking layer on the first face,
patterning the masking layer to define an etch opening at a location corresponding to the location at which the bore is to be etched into the silicon substrate,
subjecting the silicon substrate to a first dry etch through the etch opening for forming the bore to a first depth, the bore tapering inwardly from the first face to the first depth,
subjecting the silicon substrate to a second dry etch through the etch opening on completion of the first etch for etching the bore to a second depth which is a greater distance from the first face than the distance of the first depth from the first face.
In one embodiment of the invention the transverse cross-sectional area of the portion of the bore extending between the first and second depths formed by the second etch is constant.
In another embodiment of the invention the portion of the bore tapers from the first face to the first depth to be of transverse cross-sectional area adjacent the first depth similar to the transverse cross-sectional area of the portion of the bore extending between the first and second depths at the first depth.
In a further embodiment of the invention the portion of the bore extending between the first face and the first depth, and the portion of the bore extending between the first depth and the second depth are of circular transverse cross-section.
In one embodiment of the invention the portion of the bore extending between the first face and the first depth tapers to define an included cone angle in the range of 30xc2x0 to 90xc2x0.
In another embodiment of the invention the portion of the bore extending between the first face and the first depth tapers to define an included cone angle in the range of 35xc2x0 to 60xc2x0.
In a further embodiment of the invention the portion of the bore extending between the first face and the first depth tapers to define an included cone angle of approximately 40xc2x0.
In one embodiment of the invention the area of the etch opening is less than the transverse cross-sectional area of the portion of the bore extending between the first and second depths at the first depth thereof. Preferably, the area of the etch opening is in the range of 80% to 90% of the transverse cross-sectional area of the portion of the bore extending between the first and second depths adjacent the first face of the silicon substrate. Advantageously, the area of the etch opening is approximately 85% of the transverse cross-sectional area of the portion of the bore extending between the first and second depths at the first depth thereof.
Preferably, the shape of the etch opening is of shape similar to the shape of the portion of the bore extending between the first and second depths at the first depth thereof.
In one embodiment of the invention the first and second etches are carried out in a controlled environment chamber.
In another embodiment of the invention the first etch is an isotropic etch.
In a further embodiment of the invention fluorine radicals are created in the controlled environment chamber during the first etch for reacting with the silicon substrate for releasing volatile by-products for forming the portion of the bore extending between the first face and the first depth. Preferably the first etch is carried out with an etchant preparation comprising sulphur hexafluoride. Advantageously, the pressure within the controlled environment chamber is maintained in the range of 5xc3x9710xe2x88x926 bar to 2xc3x9710xe2x88x924 bar during the first etch, and the DC bias voltage on the platen is controlled by maintaining input power to the platen in the range of 0 watts to 50 watts. Ideally, the pressure within the controlled environment chamber is maintained at approximately 7xc3x9710xe2x88x925 during the first etch, and the power to the platen is maintained at approximately 5 watts.
Alternatively, the first etch is an anisotropic etch, and is carried out using the Bosch process by sequentially alternating between etch cycles and deposition cycles, and the duration of the respective etch and deposition cycles is controlled for controlling the cone angle of the portion of the bore extending between the first face and the first depth.
In one embodiment of the invention the second etch is an anisotropic etch, and is carried out using the Bosch process by sequentially alternating between etch cycles and deposition cycles, and the duration of the respective etch and deposition cycles is controlled for maintaining the transverse cross-sectional area of the portion of the bore extending between the first depth and the second depth substantially constant.
In one embodiment of the invention during the second etch each etch cycle is of duration in the range of 3 seconds to 15 seconds, and the duration of each deposition cycle is in the range of 3 seconds to 7 seconds. Preferably, during the second etch the duration of each etch cycle is approximately 6 seconds, and the duration of each deposition cycle is approximately 5 seconds.
In one embodiment of the invention in each etch cycle of the second etch fluorine radicals are created in the controlled environment chamber for reacting with the silicon substrate for releasing volatile by-products for forming the portion of the bore extending between the first depth and the second depth. Preferably, the etch cycles of the second etch are carried out with an etchant preparation comprising sulphur hexafluoride. Preferably, a passivation layer is deposited during each deposition cycle of the second etch. Advantageously, the passivation layer deposited during each deposition cycle of the second etch is a fluorocarbon polymer layer.
In one embodiment of the invention the pressure in the controlled environment chamber during the second etch is maintained in the range of 2xc3x9710xe2x88x926 bar to 7xc3x9710xe2x88x925 bar during the second etch, and the DC bias voltage on the platen in the controlled environment chamber is controlled by maintaining the input power to the platen in the range of 0 watts to 30 watts. Preferably, the pressure in the controlled environment chamber is maintained at approximately 2xc3x9710xe2x88x925 bars during the second etch, and the DC bias voltage on the platen in the controlled environment chamber is controlled by maintaining the input power to the platen at approximately 10 watts.
Ideally, a passivation layer is deposited on the surface of the portion of the bore extending between the first face and the second depth on completion of the first etch for protecting the said surface during etching of the portion of the bore extending between the first depth and the second depth by the second etch.
In one embodiment of the invention the bore is an access bore extending through the silicon substrate from the first face thereof to the second depth alt a second face of the substrate opposite the first face for providing access to a micro-machined component in an adjacent device layer, and in another embodiment of the invention the access bore accommodates an optical fibre therethrough.
Further the invention provides a semiconductor wafer comprising a substrate layer of silicon, and a device layer having a micro-machined component formed in the device layer, a corresponding access bore extending through the substrate layer for providing access to the micro-machined component, the access bore being formed by the method according to the invention.
In one embodiment of the invention the access bore is aligned with the micro-machined component.
In another embodiment of the invention the substrate layer defines a first face and an opposite second face, and the micro-machined component layer is located adjacent the second face, the access bore comprising a portion tap ring from the first face to a first depth in the substrate layer, and a portion extending from the first depth to the second face of the substrate layer of substantially constant transverse cross-sectional area.
In one embodiment of the invention an optical fibre extends through the access bore, and preferably, the optical fibre is a tight fit in the access bore for securing the optical fibre in the access bore in axial alignment with the micro-machined component.
The advantages of the invention are many. The method according to the invention facilitates the formation of a bore through a silicon substrate which has an initial tapered lead-in portion leading into a portion of substantially constant transverse cross-sectional area. The method according to the invention permits the transverse cross-sectional dimensions of the bore to be controlled within relatively tight tolerances, and where it is desired to form the two portions of the bore to be of circular transverse cross-section, the transverse cross-sectional area can be maintained substantially circular throughout the length of the bore, both in the tapering portion of the bore and in the portion of substantially constant cross-sectional area. Accordingly, by virtue of the fact that the cross-sectional dimensions of the bore can be accurately determined and maintained, the bore is particularly suitable as an access bore for aligning with a micro-machined component in a device layer adjacent to the silicon substrate, and in particular, is suitable for aligning an optical fibre with the micro-machined component. The method is particularly suitable for forming a bore of circular transverse cross-section in which the cross-sectional area of the bore is accurately maintained circular. Thus, the method permits the accurate formation of access bores through a handle layer of a semiconductor wafer, which supports a device layer comprising a plurality of micro-machined components. By virtue of the fact that the method permits the formation of accurately shaped and dimensioned bores, and furthermore, by virtue of the fact that the bores can be accurately located in the handle layer, the bores are particularly suitable for facilitating accurate alignment of optical fibres and/or other components located in the bores with corresponding micro-machined components, such as, for example, micro-mirrors. By providing the accurately formed tapered portion of the bore, a suitable tapered lead-in is provided for facilitating insertion of an optical fibre or another component or components into the bore.
A further advantage of the method according to the invention is that by forming the access bore with a relatively long and wide angled tapered lead-in, an optical or other coating can be deposited onto the micro-machined component through the bore with precision. By virtue of the fact that the cross-section of the tapered lead-in portion of the bore is accurately dimensioned, shadowing effects which would otherwise arise when depositing a coating on a micro-machined component through a relatively long narrow bore are reduced, and depending on the length and the cross-section of the bore and the angle of the tapered lead-in may be eliminated. A further advantage of the method according to the invention is that by providing the access bore with the tapered lead-in, physical restrictions to incident and reflected optical light to and from the micro-machined component is minimised.
The invention and its advantages will be more clearly understood from the following description of a preferred embodiment thereof, which is given by way of example only, with reference to the accompanying drawings.